A variety of phototropic glasses are already known. Early work in the field is disclosed in Armisteat U.S. Pat. No. 3,208,860 (German Pat. No. 1,421,838) where the base glass is a silicate or a borosilicate glass. The activating agent, i.e. the component imparting phototropy, is silver halide in the form of crystals or microcrystals.
Later, phototropic glasses in which the base glass is a borate glass, were developed. Here also the activating agent is silver halide in the form of crystals. Such glasses are disclosed in U.S. Pat. No. 3,548,060 assigned to Nippon, and Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847).
Phototropic glasses in which the base glass is a phosphate glass are also known. Sakka and McKenzie, J. Amer. Ceram. Soc. 55, 1972, 553 disclose such glasses. The activating agent or combination imparting or imparting and affecting phototropy is thallium chloride, or thallium chloride and Cu.sub.2 O, or thallium chloride and silver oxide. Those authors, in U.S. Pat. No. 3,615,761, disclose phosphate phototropic glasses in which the activating agent is thallium chloride. A description of phototropic phosphate glasses that is comprehensive in regard to the breadth of the possible compositions is available in German Offenlegungsschrift (DOS) No. 2,234,283, (equivalent to U.S. Patent of Lythgol No. 3,876,436) assigned to Pilkington Brothers, Ltd. Here also, silver halide crystals dispersed in the glass result in phototrophy. Thus, the last mentioned phototropic phosphate glasses differ from phototropic glasses on a silicate, borosilicate, or borate basis, such as are described above, only in the composition of the base glass.
Glasses of one or more oxides are composed of networks of the principal components. The principal components are bound to the network in the form of coordination polyhedra joined at the apexes. These structure units (principal components) are SiO.sub.4 tetrahedra in silicate glasses, for example; similar units of structure exist in borate and phosphate glasses. These units of structure are bounded together with different strengths in different glasses. The more strongly the central cations of these structures polarize the surrounding oxygens, the more weakly they are bonded to adjacent units of structure (e.g., to the adjacent tetrahedron). The polarization of the oxygens by the central cations is, according to Dietzel in Z. Elektrochem. 48 (1942), 9, as follows:
Table 1 ______________________________________ Valency in Distance, a, in Angstroems Field Central the Glass Between Cation and Oxy- Strength Cation Z gens, a Z/a.sup.2 ______________________________________ Si 4 1.60 1.56 B 3 1.36 1.62 P 5 1.55 2.08 ______________________________________
The greater the field strength of the central cation of such a structural unit is, the more strongly the surrounding oxygen envelope is polarized and the weaker becomes the bond of this unit of structure (consisting of the central cation and the surrounding oxygen envelope) externally to the adjacent units of structure.
Accordingly, the strength of the bond between the principal components diminishes, i.e., the overall glass structure loosens up, as one passes from silicate glasses to borate glasses or even to phosphate glasses.
In Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847), phototropic glasses are disclosed whose phototropic properties are determined by silver halide crystals and in some cases small amounts of metallic silver, and which are distinguished in that they consist of one or more glass-forming oxides as principal components whose bond to one another in the glass is weaker than the bonds in a silicate base glass with SiO.sub.2 as the glass-forming component.
In particular Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847), describes borate glasses which have better phototropic properties than the silicate glasses which were known at the time, on account of the weaker bond between the principal components.
From the above considerations, which are generally accessible to the technical world, it appears that the phosphate glasses must also be suitable for the production of phototropic glass providing a good optical density variation under the influence of impinging light rays.
If, in accordance with the teaching of Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847), the principal component B.sub.2 O.sub.3 is replaced by P.sub.2 O.sub.5, the bond between the principal components will become weaker in the resulting glass. On the basis of the examples given in the said patent we will then have the following compositions, which do have phototropic characteristics, but whose phototropy is of poorer quality. To facilitate comparison, compositions are given in parts by weight, as is done in Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847).
Table 2 ______________________________________ P.sub.2 O.sub.5 -Modified Gliemeroth 3,834,912 Glasses 1 2 3 4 5 6 ______________________________________ P.sub.2 O.sub.5 14.9 81.4 47.8 71.0 67.5 73.6 PbO 69.4 -- 35.6 -- -- -- MgO -- -- -- 15.4 13.5 -- BaO -- -- -- -- 15.4 -- ZnO 9.90 10.2 -- -- -- 9.20 Al.sub.2 O.sub.3 1.98 -- 12.5 9.60 -- -- Na.sub.2 O 0.10 -- -- -- -- -- K.sub.2 O -- 5.08 -- -- -- 7.40 KCl -- -- -- 0.48 -- 0.92 KBr 1.49 1.53 1.44 1.44 1.45 1.38 KI 1.49 1.53 1.44 1.44 1.45 1.38 LiF 0.50 -- 0.96 0.29 0.29 0.28 Ag.sub.2 O 0.19 0.30 0.29 0.38 0.38 0.37 CuO 0.005 -- 0.01 -- 0.02 0.02 K.sub.2 Cr.sub.2 O.sub.7 -- -- -- 0.01 0.005 0.01 ZrO.sub.2 -- -- -- -- -- 5.52 99.555 100.04 100.04 100.04 99.995 100.08 ______________________________________
As it has already been explained in Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847), the use of small amounts of SiO.sub.2 contributes toward stabilization. This makes the structure of the glass stronger again. In accordance, therefore, with Gliemeroth U.S. Pat. No. 3,834,912 (German Pat. No. 1,596,847), glasses are possible, in view of the above consideration, which are listed by way of example in Table 3. These glasses are listed in parts by weight to facilitate comprehension, as is done in the German patent.
Table 3 ______________________________________ SiO.sub.2 - and P.sub.2 O.sub.5 - Modified Gliemeroth 3,834,912 Glasses 7 8 9 10 11 ______________________________________ SiO.sub.2 4.49 3.10 2.99 8.55 7.46 B.sub.2 O.sub.3 6.00 5.05 4.00 7.05 7.56 P.sub.2 O.sub.5 39.33 33.75 32.73 30.19 34.62 PbO -- 5.79 27.35 -- 1.99 MgO 6.99 11.57 1.54 3.16 -- BaO 10.93 12.78 4.61 6.52 5.17 ZnO 0.44 -- -- -- -- CaO -- -- -- 6.92 4.87 ZrO.sub.2 1.75 -- -- -- 1.99 TiO.sub.2 -- -- -- 0.49 0.99 Al.sub.2 O.sub.3 17.48 12.05 9.60 21.51 18.90 Na.sub.2 O 4.81 12.15 6.45 5.93 5.97 K.sub.2 O 6.12 1.93 4.07 6.92 8.46 KCl 0.70 0.39 2.31 0.99 0.90 KBr 0.35 1.16 1.38 0.99 0.40 KI 0.09 -- -- -- -- LiF 0.36 -- 0.77 -- 0.50 Ag.sub.2 O 0.17 0.29 0.19 0.18 0.18 CuO 0.02 -- -- 0.02 0.04 KHF.sub.2 -- -- -- 0.59 -- 100.03 100.01 97.99 100.01 100.00 ______________________________________
Examples 7 to 11 also show that a certain amount of B.sub.2 O.sub.3 can be incorporated into such phototropic phosphate glasses.
Phosphate glasses are generally known for their particularly poor chemical stability. This can be counteracted by a high Al.sub.2 0.sub.3 content, but this will result in a further impairment of the already poor resistance to devitrification.
A glass which is to be used in eyeglass lenses (long-focus portion of bifocal lenses) must satisfy certain requirements in regard to commercial profitability. This means that the yield from normal production machinery and equipment must be satisfactory and the product must not be the cause of great numbers of complaints.
In the glass making apparatus commonly used today for production of glass for eyeglass lenses, glasses can be worked which assure a throughput of more than 60 kg/h. To this end the glass must be cut into portions by shearing, at a viscosity of 10.sup.3 to 10.sup.4.5 poises, in order then to be pressed into blanks. At this viscosity, which is relatively high for molten glass, many non-silicate glasses devitrify at such high rates that profitable production becomes impossible. To counteract this deficiency, it is possible to resort to viscosity reduction and to special methods of production (inasmuch as portioning by shearing is no longer possible at low viscosities), or to accept a reduction of output by temporarily interrupting production and clearing up any devitrification that may have occurred in production by briefly raising the temperature.
All of the exemplified glasses of the aforementioned Pilkington DOS No. 2,234,283 (Lythgol U.S. Pat. No. 3,876,436) which discloses phosphate glasses activated with silver halide crystals, have been melted again with this in mind and tested for devitrification. After 60 melts it is apparent that those compositions do not qualify for normal production, discussed above, on a technical scale.
The qualifications which any glass must meet for use in eyeglasses are established by the following specifications which are generally recognized by the industry:
(a) Index of refraction n.sub.d between 1.5225 and 1.5238. PA1 (b) No devitrification between 10.sup.2 and 10.sup.4.5 poises. PA1 (c) Sufficient chemical stability (characterized by resistance to hydrolysis in accordance with DIN 12,111 and by the ability to withstand the sweat test). PA1 (d) Sufficient chemical hardenability in the standard bath for normal optical crown glass (this requirement normally applies wherever strength improvement is legally prescribed for all eyeglass lenses).